Since the beginning of this century, the supply of oil energy sources has arrived at its peaks. According to the data specified in the “BP Statistical Review of the World Energy 2005”, based on the present exploitation rate, the reserves of oil in the world can be further sustained for a little more than 40 years, and the reserves of natural gas and coal can be sustained for about 67 years and 164 years, respectively. It can be expected that the liquid fuel can not be replaced during quite a long period based on the current technology of the human society. As can be seen, mankind has to face the most vital turning point in history. Meanwhile, the global warming is regarded as the prime cause of various synoptic disasters around the world in recent years and may be attributed largely to the greenhouse gas, i.e. carbon dioxide, most of which originates from fossil energy sources. Therefore, new renewable energy sources must be developed and utilized so as to ensure the continuing existence and sustainable development of human beings.
Although the process of producing biomass derived ethanol fuel by fermentation is being widely spread in the field of liquid fuel, the actual application thereof is limited by the two factors as follows: (1) the process of producing ethanol fuel from starch will consume the food for mankind, (2) the cost of the process of producing ethanol fuel from cellulose is far higher than that derived from starch. It is true that we have developed a technology for rapid pretreatment of Cellulosic biomass at room temperature under atmospheric pressure and a corresponding novel production process (PCT/CN2006/000120), the cost of producing ethanol fuel through fermentation of Cellulosic biomass is thus expected to be substantially decreased. However, since such process is limited by the current technology level, the cost of ethanol fuel produced through fermentation of Cellulosic biomass can only be lowered to a level comparable to that of starch derived ethanol fuel.
The process of producing liquid fuel from coal can be the leading trend since the reserves of coal is much higher than that of oil. Currently, there are two coal liquefaction processes for producing liquid fuel, namely, coal direct liquefaction and indirect liquefaction. However, the production conditions used in the process of producing liquid fuel by direct coal liquefaction is too rigor and only limited types of coal can be used therein. The process of producing methanol by indirect coal liquefaction is usually preferred because of the following reason: the production procedures currently used for preparing methanol fuel from synthesis gas is relatively well developed, and the conditions for coal gasification is much mild than those used for the direct coal liquefaction; the cost for producing methanol through indirect coal liquefaction is less than one third of that for producing ethanol fuel; the combustion value per unit weight of methanol fuel is 76.5% of that of the ethanol fuel; and the corrosive to engine caused by methanol can be readily resolved. For the above reasons, the development of methanol fuel will be of great competitiveness. Furthermore, we have just developed a process for producing saturated alkane (diesel oil and gasoline) from Cellulosic biomass, and there is still a long way to go before the process can be applied in the commercial scale. As can be seen, the methanol, ethanol, and the derivates thereof derived from Cellulosic biomass, coal and natural gas will necessarily become the dominant liquid fuel in the near future.
The process currently used for producing methanol through indirect coal liquefaction mainly comprises two steps as follows: in the first step, the coal is gasified to produce synthesis gas, and in the second step, the synthesis gas is converted into methanol in the presence of catalyst. Relatively developed coal gasification process mainly comprises the gasification of solid coal powder feedstock and the gasification of aqueous coal slurry feedstock. The process of gasification of aqueous coal slurry feedstock is more homogeneous and highly reliable, so the yield of synthesis can be more readily enhanced by carrying out the gasification under high pressure. Therefore, the process of the gasification of aqueous coal slurry feedstock is generally adopted, while the gasification of solid coal powder feedstock is stilled operated by some manufactures. However, the process of producing methanol through indirect coal liquefaction still possesses the following drawbacks that need to be improved:
First of all, since the coal is generally rich in carbon but lack of hydrogen, the composition of the synthesis gas derived from most types of coal is far lower than the ratio required for producing methanol (hydrogen:carbon monoxide=2:1). The methods previously used for solving above problem comprises: (1) installing a separate production line for generating hydrogen from coal so as to supplement the hydrogen; (2) installing a separate converter in the gasification process for converting carbon monoxide into hydrogen (by reacting carbon monoxide and water to produce carbon dioxide and hydrogen) so as to supplement. However, both of the above methods still include the limitations of increasing the cost and the complexity of the process, and consumption of water and coal. It is well known that the short supply of water will be more and more severe in the future. The difficulties in gasification is even more severe for some types of coal, such as the coal produced from the Eastern China, which not only are rich in carbon and lack of hydrogen but also comprise excessive ash content with high ash fusion temperature (1500° C. or even higher), etc.
Secondly, in addition to the above drawbacks, many types of coal will bring about the problem of extremely high energy consumption when used for producing methanol through indirect coal liquefaction. In the process of coal gasification, the liquid slag-tapping is generally preferred, and the liquid slag-tapping from the lower-part is more preferred for the sake of easiness in operation. The optimal operation temperature for the liquid slag-tapping furnace is usually 30-50° C. higher than the ash fusion temperature, that is, the optimal operation temperature of coal gasifying for the slag-tapping furnace is usually at least about 1550° C. when the ash fusion temperature is higher than 1500° C. or more. When carrying out the coal gasification reaction under such a high temperature, the energy consumption as well as the rate of the reaction between hydrogen and carbon to produce methanol and ethanol will be rapidly increased, and the virtual specific yield of gas will decrease. Furthermore, if the operation temperature is higher than 1400° C., the fusion corrosion rate of firebrick will double every the temperature increasing 20° C. When the operation temperature is 1550° C., the firebrick will have to be replaced more frequently because of its high fusion corrosion rate, which will severely enhance the cost of production. The conventional technical solution aimed to solve the problem is to incorporate calcium oxide (calcium carbonate), ferric oxide, or magnesium oxide (magnesium carbonate), etc. therein to decrease the ash fusion temperature. However, since the amount of calcium oxide incorporated therein is generally 20-25% based on the total amount of the ash, above solution will substantially increase the output of ash. Meanwhile, the cost will be excessively high by using sodium carbonate or potassium carbonate. Every time the ash content is increased by 1%, the consumption of oxygen and coal will increase by about 0.8% and 1.5%, respectively. Therefore, the production cost will be further enhanced and the specific yield will be further lowered. Besides, the incorporation of fluxing agent of calcium oxide will result in black water treatment and severe fouling of the thermal-exchange system, rendering great increase of the total production cost.
In additional to synthesizing methanol liquid fuel, the synthesis gas can also be used for producing other liquid fuel such as ethanol. The chemical mechanism of producing ethanol from synthesis gas is similar with the mechanism of producing methanol. Accordingly, the catalyst used for producing methanol can be used as a basis and modified to obtain the catalyst used for producing ethanol. Besides, the reaction equipments used for the process of producing ethanol may be the same with the reaction equipments used for the methanol with tiny variation, which is mainly because the process of producing ethanol will emits more heat (about 2.5 times of the process of producing methanol), thus the reaction vessel has to be modified so that the heat can be dissipated rapidly.
In addition to the eager need for improving the process of producing liquid fuel through coal gasification, the process of producing methanol through direct Cellulosic biomass gasification also encounters many issues. Firstly, the Cellulosic biomass is of low density, rendering an excessively low efficiency of the gasification furnace and high specific cost of the product; secondly, the Cellulosic biomass is rich in hydrogen and lack of carbon, rendering the excessive hydrogen in the synthesis gas can not be effectively used for the synthesis of methanol, and consequently leading to high specific cost of the production.